In recent years, the development of driving systems for motors has made frequency control of driving power possible, and the use of motors for variable speed operation or high speed rotation exceeding commercial frequency is growing. In such motors for high speed rotation, the centrifugal force applied to a rotating body such as a rotor is proportional to the rotating radius and increases proportionally to the square value of rotational speed. Therefore, it is necessary for rotor material, in particular rotor material for high speed motors of middle and large sizes, to have high strength.
Further, in an IPM (Interior Permanent Magnet)-type DC inverter controlled motor which is increasingly being adopted in driving motors for hybrid automobiles or compressor motors in recent years, a slit is provided on the outer periphery part of the rotor and a magnet is embedded therein. Because of this, stress concentrates in narrow bridge parts (e.g. parts between an outer periphery of a rotor, and a slit) due to centrifugal force during high speed rotation of the motor. Further, since the stress state varies depending on the acceleration/deceleration operation or vibration of the motor, high fatigue strength as well as high strength are required for core material used in rotors.
In addition, in high speed motors, eddy current is generated by a high frequency magnetic flux, and heating is caused as motor efficiency lowers. As this heat value increases, the magnet embedded within the rotor is demagnetized. For this reason, it is also required for the iron loss in the high frequency area to be low.
Therefore, an electrical steel sheet with high strength having excellent magnetic properties as well as excellent fatigue properties is desired as material for rotors.
As methods of strengthening steel sheets, solid solution strengthening, precipitation strengthening, crystal grain refinement strengthening, and multi-phase strengthening are known. However, since many of these strengthening methods deteriorate magnetic properties, it is generally considered extremely difficult to improve both strength and magnetic properties.
Under such situation, some proposals for an electrical steel having high tensile strength have been made.
For example, JPS60-238421A proposes a method of enhancing the strength of steel sheets by increasing the Si content to 3.5% to 7.0% and adding elements such as Ti, W, Mo, Mn, Ni, Co, and Al for solid solution strengthening.
Further, JPS62-112723A proposes, in addition to the above described strengthening method, a method of improving magnetic properties by devising conditions of final annealing and achieving a crystallized grain size of 0.01 mm to 5.0 mm.
However, when these methods were applied to factory production, there were problems such as the fact that troubles including sheet fracture were likely to occur during a continuous annealing process after hot rolling, or the subsequent rolling process and the like, and reduction in yield or line stop was unavoidable.
Regarding this point, changing the cold rolling process to a warm rolling process with the sheet temperature set to several hundred degrees would reduce sheet fracture. However, not only will it be necessary to adapt facilities to warm rolling but there are serious problems of process management including a large restriction of production.
Further, JPH02-22442A proposes a method of achieving solid solution strengthening by adding Mn and Ni to steel with an Si content of 2.0% to 3.5%, and JPH02-8346A proposes a technique of achieving both high strength and magnetic properties by performing solid solution strengthening with the addition of Mn or Ni to steel with an Si content of 2.0% to 4.0%, and using carbonitrides of Nb, Zr, Ti, V, and the like.
However, those methods have problems such as the necessity of adding a large amount of expensive elements such as Ni, or the high cost due to the reduction in yield caused by an increase of defects such as scab. Further, to date, sufficient research has not been conducted to investigate fatigue properties of materials obtained by these disclosed techniques.
Further, as a high strength electrical steel sheet focused on fatigue resistance properties, JP2001-234303A discloses a technique of achieving a fatigue limit of 350 MPa or more by controlling the crystallized grain size depending on the steel composition of the electrical steel sheet with an Si content of 3.3% or less.
However, with that method, the achievement level of the fatigue limit itself was low and could not satisfy the recently required level, e.g. a fatigue limit strength of 500 MPa or more.
On the other hand, JP2005-113185A and JP2007-186790A propose a high strength electrical steel sheet with non-recrystallized grains remaining on the steel sheet. According to those methods, high strength can be obtained relatively easily while maintaining manufacturability after hot rolling.
However, through an evaluation we performed on stability of mechanical properties of such material with non-recrystallized grains remained, we identified that the material tends to have a large variation in its mechanical properties. In other words, we identified that, although high mechanical properties are exhibited in average, even relatively small stress may cause fracture in a short time due to the large variation.
Such large variation in mechanical properties makes it necessary to improve the worst mechanical properties among varied mechanical properties, so that they have the required mechanical properties. It is understood that one method for this would be to improve the average mechanical properties. However, when material with a non-recrystallized microstructure remained, it is necessary to increase the amount of non-recrystallized microstructure by lowering the temperature of final annealing. Although this will not eliminate the variation of mechanical properties itself, troubles such as fracture can be prevented by improving relatively poor mechanical properties.
However, in the case of lowering the temperature of final annealing to increase the amount of non-recrystallized microstructure, an increase in iron loss was caused.
In other words, a large variation of mechanical properties makes an increase of iron loss unavoidable.
Therefore, reducing the variation in mechanical properties is also effective for the reduction of iron loss.
As mentioned above, by using conventional techniques under present circumstances, it is extremely difficult to stably provide a high strength electrical steel sheet having high strength, and excellent magnetic properties and manufacturability, which is a material with a small variation of mechanical strength, at a low cost.
There is, therefore, a need to provide an advantageous method of producing an electrical steel sheet stably having high strength and high fatigue properties, and excellent magnetic properties, which is suitable for use as rotor material for high speed motors.